In general, combustion apparatus used for hot water and heating, such as a boiler or a hot water heater, are classified into an oil boiler, a gas boiler, an electric boiler and a water heater depending on the fuel it is supplied with, and are diversely developed to fit different installation usages.
Among these combustion apparatuses, in particular, the gas boiler and the water heater generally use a Bunsen Burner or a Premixed Burner to combust gas fuel, and among these the combustion method of the premixed burner is carried out by mixing gas and air with mixing ratio of combustion optimum state and supplying this mixture (air+gas) to a burner port for combustion.
The function of a combustion apparatus is evaluated by the turn-down ratio (TDR). A turn-down ratio refers to ‘a ratio of maximum gas consumption versus minimum gas consumption’ in a gas combustion device in which the gas volume is variable regulated. For instance, if the maximum gas consumption is 24,000 kcal/h and the minimum gas consumption is 8,000 kcal/h, the turn-down ratio is 3:1. The turn-down ratio is controlled according to the ability to maintain a stable flame under minimum gas consumption condition.
In the gas boiler and the water heater, convenience of using hot water and heat increases with larger turn-down ratio. That is, if the turn-down ratio is small (meaning the maximum gas consumption is high) and the burner is activated for a small load volume of the heating water and heat, frequent On/Off of the combustion apparatus occurs, thereby deviation during temperature control increases and durability of the apparatus decreases. Therefore, various methods have been developed to increase the turn-down ratio applied to a combustion apparatus in order to improve aforementioned problems.
Valves which supply gas to these types of burners with proportional control are largely divided into electrical modulating gas valve, which is controlled by current value, and pneumatic modulating gas valve, which is controlled by differential pressure generated during air supply.
In the pneumatic modulating gas valve, the amount of gas supplied to the burner is controlled using a fan by differential pressure generated according to air supply needed for combustion in the burner. At this time, the air and gas needed for combustion are mixed in the gas-air mixer and supplied to the burner as a mixture (air+gas).
In a gas-air mixing device of a gas burner using such pneumatic modulating gas valve, the primary factor controlling the turn-down ration is the relationship between gas consumption (Q) and differential pressure (ΔP). The common relationship between fluid pressure and flow rate is as follows:Q=k√ΔP 
That is, differential pressure needs to be quadrupled in order to double fluid flow rate.
Therefore, differential pressure ratio must be 9:1 in order to have a turn-down ratio of 3:1, and the differential pressure ratio needs to be 100:1 to have a turn-down ratio of 10:1. However, it is impossible to infinitely increase the gas feed pressure.
In order to solve the above problem, the present invention describes, as illustrated in FIG. 1, a method for increasing the turn-down ratio of the gas burner by dividing the gas and air supply paths into more than two sections, respectively, and opening/closing each passage of gas injected into the burner.